A wheelchair seat cushion must perform a number of important functions. The seat cushion should be comfortable and capable of providing proper support for optimal posture and posture control for a considerable length of time. The seat cushion should also assist, or at least not materially hinder, the user in maneuvering the wheelchair, permit a useful range of motion from the pelvis and upper torso of the person, and create stability and security for the person within the wheelchair. Perhaps most importantly, the seat cushion should help prevent and reduce the incidence of pressure ulcers created by prolonged sitting on the cushion without adequate pressure relief. Pressure ulcers can become a very serious health problem for individuals who must remain constantly in contact with the support cushion, and it is important to avoid such pressure ulcers.
Wheelchair users like everyone are of substantially different sizes, weights and shapes. Many wheelchair users have physical disabilities and associated posture and postural control impairments such as those typically caused by congenital disorders. Other wheelchair users, such as those who have been disabled by acquired or traumatic injuries, may have a more typical size and shape. In all of these cases, the support contour of the wheelchair seat cushion must safely support the anatomy of the user, whether the anatomy is abnormal or more typical. Wheelchair seat cushions must fit and perform properly to prevent further physical impairment and pressure ulcers. The cushion must also enhance the functional capabilities of the user by supporting independence in activities of daily living.
To provide the best individualized support, the cushion must accommodate the anatomical particularities and preferences of the user. Custom wheelchair cushions are used for this purpose. A custom wheelchair cushion is created from an impression of the anatomy of the user. After capturing a shape of the user's anatomy, the captured shape is used to construct a mold for the cushion. Then the mold is used to fabricate the cushion, including the support contour which interfaces with the user's anatomy from which the shape was originally captured. There are a number of different theories for configuring the support contour to address the perceived needs and requirements of the user.
The most prevalent approach used to configure the support contour of a custom cushion, at least at the time of filing hereof, is to distribute the weight of the user substantially uniformly over the entire support contour. The uniform pressure distribution is theorized to reduce the incidence of pressure ulcers because the uniform pressure distribution is thought to avoid localized high-pressure points which cause pressure ulcers. The substantial conformance of the support contour to the anatomical shape of the user is also believed to encourage the user toward proper postural alignment.
A new support theory is described in the above-identified U.S. patent application Ser. No. 10/628,860. This new support theory is based on offloading and isolating pressure and shear forces from the skin surrounding the bony prominences of the user's pelvic area skeletal structure. Applying this support theory involves configuring the support contour with additional clearance, and therefore achieving greater pressure relief, around the ischial tuberosities, the greater trochantors, the coccyx and the sacrum in the pelvic area, while transferring more support to the broader tissue and musculature below the proximal thigh leg bones and at the posterior lateral buttocks. Pressure and shear forces on the skin around the bony prominences is relieved, and pressure is transferred to the broader tissue areas to encourage proper postural alignment.
The effectiveness of implementing any of the different support theories depends on evaluating the fit of the support contour. The user may offer comment about the feel of the support contour. However, in some cases the user is unable to offer meaningful comments, because the aspects of the fit may not be apparent to the user except in extremely exaggerated circumstances. Some wheelchair users may not have the neurological capacity to feel those areas of their anatomy which contact the seat cushion. It is common for a trained assistant to insert his or her fingers between the user and the support contour to evaluate the degree of clearance, but such an approach generally does not provide an objective evaluation. Moreover, certain areas of fit, such as the area directly under the user at the ischial tuberosities cannot be felt at all, because this location is too far underneath the user to be reached. For the evaluation to be effective, the user cannot move from a normal sitting position, because to do so alters the entire interaction of the anatomy with the support contour to such a degree that no aspect of the fit is normal. Moving to an abnormal position against the support contour to provide space for finger evaluation is therefore meaningless.
Pressure mapping has also been used to evaluate the fit of a person with the support contour of a seat cushion. Pressure mapping requires the use of a blanket-like device having hundreds or thousands of pressure sensors distributed in a grid-like manner over the entire surface. Each of the pressure sensors is connected by electrical conductors. The blanket-like pressure mapping device is placed on top of the support contour, and the individual is seated on top of the mapping device. The pressure sensors of the mapping device are located between the support contour and the anatomy of the individual. By individually reading the pressure measurements of each sensor, and correlating the positions of the sensors relative to the support contour, the pressure distribution over the entire support contour can be evaluated. Any areas of increased pressure, where pressure ulcers may ultimately occur, are accurately identified.
Such pressure mapping devices are quite effective. However, they are also expensive and require auxiliary computer support equipment to correlate the individual pressure measurements to positions on the support contour and to evaluate the pressure distribution over the support contour. The vast majority of providers and prescribers of specialized seat cushions do not have access to pressure mapping devices. Pressure mapping is costly and requires a degree of training to become competent in its use.
While such pressure mapping devices are effective in evaluating relative pressure, they are not effective in measuring the extent or degree of clearance. In general, an indication of a lack of pressure is a suggestion of some amount of clearance at a particular location between the anatomy and the support contour, but the extent of the clearance or separation is not indicated by a relative lack of pressure.
Some support theories are primarily dependent on clearance, rather than pressure. One such support theory is described in the above-referenced U.S. patent application Ser. No. 10/628,860. This support theory requires sufficient clearance at locations where pressure is completely offloaded from the bony prominences of the pelvic area of the user, and maintenance of that clearance during acceptable changes in posture of the user, during normal ranges of user movement. The clearance should also accommodate a reasonable level of tissue change or atrophy over time. Under these circumstances, the degree or amount of clearance becomes a very important variable. The degree of clearance relates to the ability of the support contour to accommodate or compensate for the range of posture changes, normal movement and tissue and musculature atrophy before those changes become so significant that the clearance disappears and the risk of pressure ulcers arises. An indication of a relative lack of pressure under one postural, movement or tissue condition may not be a reliable indication of sufficient clearance to avoid pressure and shear forces on the tissue under other dynamic conditions. A pressure mapping device is not entirely useful to evaluate the clearance relationship of the user's anatomy relative to the support contour of the seat cushion, under these circumstances.
Many of the same considerations also apply with varying levels of criticality to other uses of seat cushions. For example, seat cushions used in office environments are required to support the user in a comfortable manner which encourages proper posture and without creating risks of medical problems, for example inducing blood circulatory problems. Evaluating the suitability of a support contour of an office chair to an office worker, or a similar situation is within the scope of the present invention.